This invention relates to an art for supplying voltage to a starter circuit for applying a start signal to a discharge lamp for starting the discharge lamp in a discharge lamp lighting circuit.
The configuration of a lighting circuit of a discharge lamp, such as a metal halide lamp, comprising a DC power supply circuit, a DC-AC conversion circuit, and a starter circuit is known. For example, in the configuration wherein a DC-DC converter is used as a DC power supply circuit and a full-bridge type circuit comprising two pairs of semiconductor switch (or switching) elements for performing switching control and a driver circuit thereof are used for a DC-AC conversion circuit, the positive-polarity voltage (positive voltage) output by the DC-DC converter is converted into rectangular-wave voltage by the alternating operation of the full-bridge type circuit, then this voltage is supplied to a discharge lamp.
For the discharge lamp starter circuit, a method of increasing primary voltage generated in a primary circuit of a transformer (starter transformer) by the transformer and applying the increased voltage to the discharge lamp is known. However, how supply voltage to the primary circuit is generated becomes a problem. For example, the following methods can be named:
(1) Method of providing the primary voltage from output voltage of the DC power supply circuit or the DC-AC conversion circuit;
(2) method of providing the supply voltage (primary circuit voltage) by increasing output voltage of the DC-AC conversion circuit through a voltage doubler circuit, etc.,;
(3) method of providing the primary circuit voltage by adding a winding to the secondary side of a converter transformer placed in the DC power supply circuit and rectifying and smoothing output of the secondary winding.
First, the method (1) is intended for using the output voltage of the DC power supply circuit intact.
In the method (2), the output voltage after being increased through the voltage doubler circuit consisting of a diode and capacitor can be used as the primary circuit voltage. That is, a first capacitor and a switch element are provided in the primary circuit and the charges accumulated in a second capacitor are transferred to the first capacitor, whereby the terminal voltage of the first capacitor is raised. When the terminal voltage of the first capacitor reaches threshold voltage and the switch element conducts (or breaks down), the generated voltage may be increased by the transformer.
In the method (3), aside from a secondary winding for power output to the discharge lamp, a secondary winding is added to a converter transformer provided in the DC power supply circuit as the DC-DC converter, and the primary voltage can be used as the primary voltage to the starter circuit.
To light a discharge lamp more reliably, the voltage applied to the discharge lamp needs to be set to a reasonably high voltage (overcurrent voltage) temporarily before the discharge lamp lights up. The reason is as follows: When a start pulse generated by a starter circuit is applied to the discharge lamp and the discharge lamp breaks down, the tube voltage of the discharge lamp lowers, so that charges of a smoothing capacitor in a DC power supply circuit or charges of a capacitor in a current auxiliary circuit (for example, refer to JP-A-9-223591) provided at a later stage of the DC power supply circuit become an electric current to the discharge lamp and the reliability of the transition to ark discharge can be enhanced.
By the way, the above-described methods involve the following problems:
First, in the method (1), there is a tendency to raise the voltage increase ratio of the starter transformer and unless the inductance of the secondary winding of the transformer is made large, a start (pulse) signal of a sufficient crest value cannot be provided, thus upsizing the transformer and an increase in costs become problems.
In the method (2), alternating output of the full-bridge type circuit in the DC-AC conversion circuit before the discharge lamp is lighted (under no load) becomes a problem. That is, as well known, if the polarity of the supply voltage when the discharge lamp breaks down by a start signal is always limited to defined polarity, the transition of the discharge lamp to ark discharge can be made stably. This means that the bridge alternating operation should not be performed before the discharge lamp is lighted. However, the voltage doubler circuit is a circuit configured based on the premise that the alternating operation of the full-bridge type circuit is performed. (For example, the second capacitor is charged during the positive voltage period of a square wave provided from the output terminal of the full-bridge type circuit, and charges are transferred from the second capacitor to the first capacitor during the negative voltage (or ground) period of square wave, then the terminal voltage of the capacitor is raised. Thus, it runs counter to the lighting method of fixing the polarity of the supply voltage to the discharge lamp to one polarity before the discharge lamp is lighted.
In the method (3), the transformer in the DC-DC converter is provided with one additional secondary winding and it becomes necessary to raise the withstand voltage for the transformer, thus the transformer is upsized and costs are increased.
It is therefore an object of the invention to simplify the circuit configuration for supplying voltage to a starter circuit in a discharge lamp lighting circuit and reduce the costs of the lighting circuit.
To the end, according to the invention, there is provided a discharge lamp lighting circuit comprising a DC power supply circuit for receiving DC input voltage and outputting any desired DC voltage, a DC-AC conversion circuit being placed at the stage following the DC power supply circuit for converting the output voltage thereof into AC voltage and then supplying the AC voltage to a discharge lamp, and a starter circuit for generating a start signal to the discharge lamp, superposing the start signal on the output voltage of the DC-AC conversion circuit, and applying the resultant signal to the discharge lamp. In the discharge lamp lighting circuit,
(a) the starter circuit has a transformer and a secondary winding of the transformer is connected at one end to an output terminal of the DC-AC conversion circuit and at an opposite end to the discharge lamp;
(b) a primary circuit containing a primary winding of the transformer is provided with a first capacitor and a switch element and when the switch element conducts, the first capacitor is discharged and generated voltage at this time is increased by the transformer, then is applied to the discharge lamp via the secondary winding of the transformer;
(c) a second capacitor placed at the output stage of the DC power supply circuit is charged when the output voltage of the DC power supply circuit is equal to or greater than one threshold value; and
(d) a third capacitor is placed at the stage following the DC power supply circuit and a cycle is repeated wherein when the output voltage of the DC power supply circuit is less than the threshold value, charges accumulated in the second capacitor are transferred to the third capacitor and when the output voltage of the DC power supply circuit is equal to or greater than the threshold value, charges accumulated in the third capacitor are transferred to the first capacitor, whereby terminal voltage of the first capacitor is raised and then finally reaches a sufficient voltage value for the switch element to conduct.
Therefore, according to the invention, a cycle is executed wherein the charge transfer from the second capacitor to the third capacitor and the charge transfer from the third capacitor to the first capacitor are repeated, whereby the terminal voltage of the first capacitor is raised and when the voltage reaches a sufficient voltage value for the switch element to conduct, the start signal is applied to the discharge lamp via the second winding of the transformer, so that the circuit configuration for providing supply voltage to the starter circuit can be simplified.